trait Animal{
    fn baby_name() -> String;
}
struct Dog;
impl Dog {
    fn baby_name() -> String {
        String::from("Spot")
    }
}
impl Animal for Dog {
    fn baby_name() -> String {
        String::from("puppy")
    }
}
#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_trait_override() {
        assert_eq!("Spot", Dog::baby_name()); //Dog关联方法
        assert_eq!("puppy", <Dog as Animal>::baby_name()); //调用Dog的Animal trait的关联方法。使用全限定名
        //<Type as Trait>::fn(); 调用方式。
    }
}